This invention relates generally to wireless communication systems, and more particularly, to filters for pulse shaping in wireless communication systems.
Wireless communication systems are increasingly required to handle more voice capacity. Multimedia applications are also requiring additional transmission capacities. This increased required capacity has resulted in a need for higher spectral efficiency, which is essentially the amount of information that can be transmitted over a given bandwidth in a digital communication system. The spectral efficiency is a measure of the efficiency of the use of a frequency spectrum by a physical layer protocol. Accordingly, the spectral efficiency is the channel capacity or maximum throughput of a communication link when a particular communication method is used, for example, when a particular modulation scheme is used.
In response to this need for higher spectral efficiency, higher order modulation schemes, as well as improved channel coding techniques have been developed. In digital communications, a digital signal is modulated onto an analog carrier frequency using a keying scheme, for example, a Gaussian-filtered Minimum Shift Keying (GMSK) scheme. A GMSK scheme is a type of continuous phase frequency-shift keying that has a high spectral efficiency and that may be used, for example, in a Global System for Mobile Communications (GSM) network or system, and which may be referred to generally as GSM keying.
Known systems for providing GSM keying use a sample rate conversion to, for example, upsample the signal. The upsampling is performed using signal taps of a filter. The number of signal taps increase as the length of the filter providing the GSM keying increases, for example, when operating at higher frequencies. As the number of taps increase to provided operation at the higher frequencies, the number of components associated therewith also must increase. For example, the number of multipliers that are used to process the signals from each tap also must increase. Accordingly, at this higher frequency operation with more taps, power consumption is increased to operate all of the additional components, thereby increasing power consumption requirements and reducing, for example, battery life. Further, the complexity of the controls also increases. Additionally, the space, for example, in a chip, to accommodate these additional components increases, which can increase the overall size of the chip. Thus, the size of the case for the chip also may have to be increased, resulting in a physically larger overall device, which is undesirable in mobile devices.